Method of producing a skin-covered foamed thermoplastic

ABSTRACT

Method of producing foamed thermoplastic material having a lowdensity core and at least one integral, high-density, thin skin, the material being suitable for the production of threedimensionally formed articles on a continuous basis. A narrow strip of the thermoplastic material is extruded at a high linear rate from an arcuate mouth die, immediately surface-chilled to produce the skin, and laterally expanded over a curved mandrel to avoid distortions such as wrinkling. Operating parameters are critically controlled.

United States Patent Winstead 5] June 13, 1972 [54] METHOD OF PRODUCINGA SKIN- 3,317,363 5/1967 Weber ..264/32l X COVERED FOAMED THERMOPLASTIC3,391,05l 7/1968 Ehrenfreund et a1. ..264/48 X 3,422,172 1/1969 Dekker..264/48 X [72] Inventor: Thomas W- Winstead, 2 Overlook L n 3,426,1112/1969 Simpson ..264/48 Baltimore, Md. 21210 FOREIGN PATENTS ORAPPLICATIONS 22 Filed: Dec. 24, 1969 291,940 7 1965 N th 1 ds ..264 5121 App1.No.: 887,982 I e Related Application Data PrimaryExaminer-Philip E. Anderson Attorney-Raphael Semmes [60]Continuation-impart of Ser. No. 736,871, April 2,

1968, abandoned, which is a division of Ser. No. [57] ABSTRACT 506,805,Nov. 8, 1965, Pat. No. 3,461,496.

Method of producing foamed thermoplastic material having a 52 us. c1...264/48 18/12 TT 161/160 OW-density and least integral highdensity,thin 02 2 5 77 R 564/210 R 2 37 skin, the material being suitable forthe production Of three- 264/1313 dimensionally formed articles on acontinuous basis. A narrow 51 1m. (:1. ..B29d 7 04, 1329a 7 20, 1329a7/24, Strip Ofthe thefmplastic matexial is extruded a high line 329d27/00 rate from an arcuate mouth die, immediately surface-chilled 58Field of Search ..264/48, 51, 53, 54, 321, 177 R, to Produce the Skin,and laterally expanded Over a curved 2 2 0 R, 237, DIG. 18/12 r;161/160402 mandrel to avoid distortions such as wrinkling. Operatingparameters are critically controlled. [5 6] References Cited UNITEDSTATES PATENTS 4 Claims, 2 Drawing Figures 3,311,681 3/1967 Cherney etal "264/48 PATENTEDJun 13 m2 3, 670.059

I l5a.

FIG./ 10-. 5

uvvewran THOMAS M WINSTEAD BY ATTORNEY METHOD OF PRODUCING ASKIN-COVERED FOAMED THERMOPLASTIC REFERENCE TO CO-PENDING APPLICATIONSThis application is a continuation-in-part of Ser. No. 736,871, filedApr. 2, 1968, now abandoned, which is a division of Ser. No. 506,805,filed Nov. 8, 1965, now US Pat. No. 3,461,496, issued Aug. 19, 1969.Reference is also made to co-pending application Ser. No. 475,734, filedJuly 29, 1965 now U.S. Pat. No. 3,387,328, issued June 11, 1968, andapplication Ser. No. 506,804, filed Nov. 8, 1965, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method ofproducing thermoplastic material having a low-density core and at leastone, thin, high-density, integral skin of the same material, which maybe provided continuously over either or both of the opposite majorsurfaces of the core or at selected regions thereof. Low-density asemployed herein means a density within the range of from less than 1 upto 4 pounds per cubic foot, and very preferably less than about 3 poundsper cubic foot. High-density means a density of from about 50 to about100 pounds per cubic foot, or substantially the same density as theresin stock prior to extrusion.

Prior techniques are incapable of economically producing low-densityfoamed thermoplastic material with thin, smooth, integral, high-densityskin, because of limitations affecting various phases of theconventional processes. Consider first the conventional extrusion dies.When foamed plastics are extruded from a flat slit die, the resultingextrudate immediately expands upon emergence from the die lip. Thisexpansion is threedimensional, and the amount of expansion dependsfundamentally upon the resulting density. For example, if a resin isextruded which has an original density of 60 pounds per cubic foot andby the addition of cells the density is reduced to three pounds percubic foot, the extrudate expands about 2.7 times in each dimension asit emerges from the die. Assuming for the moment that the expansion inthickness of the extrudate is of little concern (since the thicknessdimension of the sheet is relatively small to begin with) and thatlongitudinal expansion is of little concern (since the rate at which theextrudate is taken off can readily be made 2.7 times the take-off speedwhich would prevail if the material were not expanding), there stillremains the problem of the width dimension which increases 2.7 times.For example, if the extrudate is three inches wide before the expansionat the die lip, the width after expansion is over 8 inches. At thecenter line of the die orifice, this is of no particular concern,inasmuch as the center line continues to remain the center line as thematerial expands and is taken off. However, the edge of the extrudate,in the case of a fiat slit die,must move rapidly from a point 1-% inchesfrom the center line to a point of over 4 inches from the center line.Since most of the expansion in a cellular material occurs very close tothe die orifice, the geometry of suitable apparatus to cope with thiswidth-wise expansion problem is critical. If this expansion is notproperly accommodated, the extrudate will corrugate or will wrinkle(particularly low-density foam extrudate provided with a skin bychilling), resulting in poor quality or an unusable product. Thus, wherea flat, smooth, skin-covered, low-density sheet is required, the needfor appropriate accommodation of expansion to prevent wrinkling becomesof utmost importance. The flat slit die fails in this respect.

In tubular extrusion of foam sheet, the wrinkling or corrugating problemis controlled to some degree by the bubble method". As the tube emergesfrom the die orifice, it is expanded by the internally trapped air, orsometimes by an internal shoe, which helps to remove the wrinkles orcorrugations. However, this method has geometric limitations whichpreclude the extrusion and spreading of low-density foam and thecollapsing or slitting of the tubular extrudate close enough to the dieand soon enough to permit subsequent operations while the extrudate isstill sufficiently pliable. Furthermore, as

a practical matter, it is not possible to provide a tubular extrudatewith an integral skin at one side only, because the tube becomesuncontrollable if an attempt is made to cool only one portion of theentire circumference.

More important, conventional chilling techniques are ineffective toproduce a smooth, thin, integral skin upon low-density foam, because thechilling takes place after expansion of the surface has already begunand merely shrinks expanding cells, rather than preventing cellexpansion at the surface of the extrudate altogether. The prior art hasbeen compelled to utilize expensive laminating techniques forskin-covered foam production, but such techniques introduce their ownproblems.

BRIEF DESCRIPTION OF THE INVENTION It is accordingly a principal objectof the present invention to provide an improved method for producingskin-covered thermoplastic material, specifically thermoplastic materialhaving a low-density core and at least one high-density integral skin ofthe same material.

Briefly stated, in accordance with the preferred embodiment of thepresent invention, thermoplastic material, such as polystyrene resinstock, for example, heated to an appropriate temperature and mixed witha volatile blowing agent, is extruded under high pressure from a curvedlip die along radii of the die lip curvature, so as to produce a fiat,narrow-strip, lowdensity foamed extrudate at high linear rates. At leastone surface of the extrudate is chilled to a critical degree at alocation critically close to the die lips to produce a thin,high-density skin. The extrudate is then spread laterally over a curvedmandrel. Critical operating parameters permit the production of highquality, skin-covered .foamed thermoplastic material without adverselyaffecting the density and cell structure of the foam core and withoutintroducing wrinkles or other im perfections in the integral skin.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be furtherdescribed with reference to the accompanying drawings, which illustratetypical apparatus for performing the process of the invention, andwherein:

FIG. 1 is a longitudinal sectional view illustrating a typical extruderhead, chiller, and mandrel employed in performing the method of theinvention; and

FIG. 2 is an end view showing the extrusion die and mandrel andillustrating an extruded sheet of foamed thermoplastic passing over themandrel from the die lips.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, 5 representsan extruder body having a longitudinally extending central passageway 6fed from a suitable source of molten plastic (not shown) under pressure.The thermoplastic raw material may be a composition comprising polymersof ethylenically unsaturated monomers, such as polystyrene,polyethylene, polyvinylchloride, or polypropylene, for example generalpurpose styrene containing 15 percent by weight of a non-inflammablevolatile liquid blowing agent, such as trichlorofluoromethane (orpentane), and a suitable nucleating agent, such as 1 percent by weightof inorganic tale. The discharge end of the passageway or barrel 6passes through an extruder head 7 and preferably terminates incommunication with a converging inlet port 8 located centrally of acircular die plate 9. An end cap 10, also circular in shape, registerswith the plate 9, being secured in place by any suitable means, and theinner face of the end cap is conically recessed, as at l 1, providing,with the plate 9, a feed chamber 12 which is coaxial with the passageway6 and converging opening 8. The conical front wall of the chamberterminates slightly inward of the periphery of the end cap 10 and iscircumscribed by a narrow annular wall at the periphery. An adapter 13conforms substantially to the surface contour of the chamber 12, withthe exception that the upper portion thereof is provided with a V-shapedcut-out area which,

when the adapter is installed, provides upwardly diverging radialconfines for the chamber 12, thereby providing a substantiallyfan-shaped die reservoir, the upper portion of which terminates incylindrical lips 1212 (see FIG. 2), all as shown in the said co-pendingapplication Ser. No. 475,734 now U.S. Pat. No. 3,387,328. The arc-lengthof the cylindrical die opening is chosen to produce a narrow-stripextrudate, which will be considered later in detail.

Also, as described in the said copending application Ser. No. 475,734now US. Pat. No. 3,387,328, to ensure the controlled spreading of theextrudate after leaving the: die lips, a mandrel or spreader 14 isprovided adjacent the die lips. This mandrel preferably comprises avertically disposed, transversely arcuate tube which is supported in anupright position, so that as the extrudate 15 leaves the die lips, it isspread laterally and is supported so as to provide a maximum possibleaccommodation of the width-wise expansion of the extrudate. See FIG. 2.

As already noted, the extruded material emerges from a cylindrical dieface rather than a flat die face. Internally within the die, thematerial flow begins at a point which is equidistant from all points onthe cylindrical die lips 12b. This provides a uniform distribution ofpressure and flow at all points along the lips and thereby eliminatesstrains or distortions in the extrudate. As the extrudate emerges fromthe orifice and expands, it continues to move radially outward to themandrel or spreader. This geometry provides, again, the maximum possibleaccommodation of the width-wise expansion. The extruded strip is thenpassed over the mandrel and may be drawn away at an oblique or acuteangle with respect to the plane of the mandrel. If the mandrel isconcentric to the cylindrical die opening, nothing further is gained bytaking away the extrudate from the mandrel at more than a 90 angle tothe plane formed by the mandrel. However, if the take-away angle is lessthan 90, becoming acute with respect to the plane of the mandrel or morenearly parallel to the mandrel plane, the shape of the mandrel may thenbe made other than concentric with the die opening, either by making itelliptical in shape, rather than circular, or by moving it closer to.the die opening, which effectively does the same thing. In practice, theextruded sheet may be drawn away from the mandrel in a direction almostparallel to the plane of the mandrel, that is, almost opposite to thedirection of extrudate flow from the die opening to the mandrel.

In the space between the cylindrical die orifice and the mandrel, thecorrugations or wrinkles of an expanding cellular sheet or strip arevery efficiently and quickly removed. It will be noted that the mandrelis hollow in order to provide for circulating coolant, and its surfacemay be coated with an antifriction surface coating. The flow of materialover the mandrel not only serves to remove corrugations or wrinkleswhich would otherwise form upon expansion of the extrudate, but also, ifproportioned properly and positioned properly, the mandrel can actuallyprovide transverse stretch to the extruded sheet as it is pulledlongitudinally around the mandrel. This has the advantage of providing acertain amount of biaxial orientation, which is desirable with certaintypes of materials to improve physical properties.

To provide an integral skin upon the foam extrudate core, an arcuatehousing 16 is mounted immediately adjacent the discharge orifice fromthe extrusion chamber 12a. Cooled air or other gas is fed to the housingthrough tube 17 from a suitable source (not shown). The housingpreferably converges at its discharge end to form an arcuate dischargeslot 18 (generally following the contour of the arcuate die lips) whichis directed against the adjacent surface of the extrudate 15.Preferably, a baffle 19 is provided in the housing 16 to facilitateuniformity of the flow of the cooling medium.

As the extrudate l progresses from the extrusion chamber 120, it expandsrapidly. It is essential, in accordance with the present invention, thatthe surface to be provided with a skin be chilled very rapidly to atemperature at which the chilling precludes any substantial expansion ofthe chilled surface, and

does not merely shrink the cells at the surface or prevent their fullexpansion to a sphere. Thus, although the extrudate 15 expands rapidlyas it emerges from the die, the chilled surface remains unexpanded andprovides a molecularly integral thin skin 15a for the foam core. Thethickness of the skin is directly controlled by the rate of flow andtemperature of the cooling medium and by the rate of extrusion.

The method of the invention is applicable to thermoplastic extrudatescontaining undissolved blowing agents (such as gas in small spheresunder pressure) and is particularly applicable to thermoplasticextrudates containing dissolved blowing agents, such as, for example,styrene containing pentane. In the former case, the chilling preventsthe expansion of cells near the surface of the extrudate by cooling thesurface, just as the extrudate emerges from the die, to a sufficientlylow temperature as to cause such an increase in viscosity of thethermoplastic that it will not flow to accommodate cell growth even asthe pressure on the extrudate is reduced to atmospheric. In the lattercase, where a dissolved blowing agent has been incorporated, the methodprovides for chilling of the surface in a similar manner, but thedissolved blowing agent is prevented in the first place from forming gascells because the increase in viscosity and decrease in temperature atthe surface of the extrudate actually reduce the boiling point of theincorporated blowing agent to a level where it remains in its liquidphase. Typically, the chilled temperature of the skin is about 220 to235 F.

As stated above, chilling of the surface of the extrudate to form theskin must take place at a location extremely close to the die lips. Yet,the chilling must be isolated from the die lips. Thus, the coolinghousing 16 is located very close to the die lips 12b, but an air space16a is provided between the housing 16 and the extrusion head tomaintain the proper cooling temperature in the housing 16 and to avoidcooling the extruder head and die. The slot 18 is designed to facilitatethe discharge of a high velocity uniform flow of air or other coolinggas upon the adjacent surface of the extrudate, the amount of gas, itsrate of flow, temperature, and the rate of extrusion being variableswhich contribute to the thickness of the skin. The skin must be thinenough to permit stretching during expansion of the foam core and duringthree-dimensional forming which may follow without interruption.

In the embodiment of the die shown in FIG. 1, the cooling space 16a isrelied upon to insulate the extrusion head from the cooling unit. Thismay also be accomplished by providing a thermal barrier between thesetwo units. It is important that this thermal barrier or the insulatingair space be effective to maintain a sharp temperature gradient betweenthe lips of the die and the skin-producing element. In a typicaltemperature profile, the die body 7 is maintained at approximately thesame temperature as the extrusion stock, for example, 300 F., while thecooling unit 16 and the cooling medium are maintained at a temperatureof perhaps 0 F. The temperature of the cooling unit 16 may be controlledby any conventional temperature-regulating device.

Where desirable, the skin may be provided at selected regions of asurface of the extrudate, rather than continuously covering the surface,by controlling the cooling pattern of the cooling medium dischargedthrough the housing 16 (or other cooling unit) or by applying thecooling medium to selected areas of a surface of the extrudate. It isalso possible to form skin on selected areas by controlling the intervalof application of the cooling effect time-wise in relation to the rateof extrusion. A checker board effect may be produced in this manner, forexample. Skin may be formed on either or both major surfaces of theextrudate, an additional cooling unit, such as unit 16, being providedat the opposite side of the extrudate in FIG. 1 if a double skin isdesired.

As indicated in the said co-pending application Ser. No. 506,805 now US.Pat. No. 3,461,496, the cooling of the surface or surfaces of theextrudate may be effected by conductive cooling instead of by convectivecooling, and in the former case, it is desirable to coat the conductivecooling surfaces with an anti-friction polytetrafluorethylene.

The cooling effect on the surface of the extrudate does not materiallyaffect the temperature of the foam below the surface, because of theexcellent thermal insulating quality of the low-density foam. As aresult, after chilling of the surface of the extrudate, 90 percent ormore of the total thickness of the foam remains sufficiently hot andpliable for subsequent forming operations, if such operations areperformed quickly enough, even though the skin surface has been chilledsufficiently to prevent its expansion. Because the skin is relativelythin, it can be stretched in such forming operations in spite of thefact that its temperature may actually be below optimum forming levels.Also, a high degree of biaxial orientation of the skin molecules may beproduced and maintained.

As the extrudate emerges from the die, it expands rapidly, because theblowing agent, which previously has been in solution due to pressures inthe system of perhaps 500 psi or more, is no longer under more thanatmospheric pressure (approximately psi) and, at the lower pressure,comes out of solution and volatilizes. This change of state naturallyabsorbs heat energy, thus rapidly lowering the temperature of theextrudate. The decrease of temperature, which is especially pronouncedin low-density foam, is further accelerated by heat loss throughradiation and convection. When the extrudate surface is then chilleddeliberately to impart a skin thereto, heat loss by convection orconduction chilling further reduces the temperature of the material. Itmight be assumed that stock temperatures coming from the extrusion diecould be increased in order to compensate for there the loss of heat.However, with foam materials there is limit beyond which the stocktemperature cannot be increased without causing cell collapse and/orbrittleness. For example, with styrene, this limit is of the order of300 F. in the die chamber.

It has been discovered, in accordance with the invention, that theproblem of heat loss from the extrudate (which may be crucial tooperations subsequent to extrusion) can be solved by the production of athin-strip extrudate at high feed rates, and that other importantadvantages can be provided as well. Whereas, generally, one mightsuppose that it would be best to extrude a fairly wide sheet in order tohave a web width which would accommodate, for example, a number ofcavities to be produced by subsequent forming operations, just theopposite is desirable in accordance with the invention. The difficultiesof removing corrugations from a wide flat sheet after emergence from thedie are almost insurmountable in lowdensity foams. Moreover, the slowlinear speed generally associated with wide webs inherently includes atime factor which is detrimental to operations subsequent to extrusion.

When a narrow strip is used, as in the present invention, corrugationsor wrinkles in the web occuring at the lower densities can be overcomeby stretching the material to a greater are distance of the mandrel.This is not possible with a wide or tubular extrudate until after damagecaused by low-density corrugations has already resulted. This problem isparticularly important with skin-covered materials, as the skin itselfreflects these variations in the finished products surface. (The choiceof blowing agent afiects the skin gloss; e.g., a blowing agent such astrichlorofluoromethane will produce a much higher skin gloss on styrenethan dichlorodifluoromethane, in which styrene is less soluble.)

The high linear rate of feed is an important factor in eliminating thepossibility of crushing of the strip as it passes over the spreader ormandrel. A wider sheet, and the inherently slower linear rate resultingtherefrom, would make cell collapse a definite difficulty. However, inthe method of the invention, the extruded material is still expanding asit reaches the mandrel, and its temperature is well above the boilingpoint of any volatile blowing agents which might be used, thuseliminating the problem. The cells, having retained much of the volatileblowing agent, are still under reasonable internal pressure, making themresilient enough to resist crushing.

material, such as The high linear speed of the narrow-strip extrudate ofthe invention is also important to the proper formation of the skin. Ifextrusion speeds are too slow, expansion will take place at the veryedge of the die lip or even within the die lip itself, thus preventingthe application of cooling medium until it is too late to preventexpansion of the surface. For example, it has been found that at anoutput rate of 200 pounds styrene extrudate per hour, a density of 2.5pounds per cubic foot, a final thickness of 0.125 inch, a stocktemperature of 300 F and a final strip width of 6 inches, the expansiondoes not occur until a point about 0.100 inch from the die lip. Thisprovides ample space to direct a chilling medium against the surface toprevent expansion and form a skin. On the other hand, if the web widthis doubled, all other variables remaining the same, the availabledistance from the die lip for adequate chilling becomes so short that itis almost impossible to achieve a good skin.

The extruded strip, in accordance with the invention, is optimallybetween 0.020 and 0.300 inch thick after expansion, with an integralskin of thickness between 0.001 and 0.005 inch. The extruded strip isoptimally within the range of from less than 1 to a maximum of 18 incheswide after expansion, the arc length of the die orifice being aboutone-third the expanded width of the strip for low-density foam. It hasbeen found that chilling of the surface of the extrudate to form theskin must commence at a location no more than about 0.050 inch from thedie lips and preferably within the range of 0.015 to 0.050 inch. Atypical temperature profile beginning with the stock in the die is shownin the following table:

TABLE I Die stock temperature 300 F. A" from die lip 280 F. At mandrel270 F.

Compared to prior art extrusion-forming methods, the feed rate of theextrudate produced by the invention is very high. Assuming a density of2.5 pounds per cubic foot and a final thickness of the extrudate of0.125 inch, for example, the following linear extrudate feed rates aretypical for the stated widths and pounds per hour of extrudate produced:

TABLE II No./hr. 200 No./hr. 300 No./hr.

6 width 128 ft./min. 256 ft./min. 384 ft./min. 12 width 64 ft./min. 128ft./min. 192 ft./rnin.

The minimum feed rate is of the order of 40 feet per minute.

The following table gives typical operating conditions in accordancewith the invention:

In a typical system for carrying out the method of the invention, themandrel is located between 2 and 3 inches away from the die lip.

From the foregoing, it is believed that the invention may be readilyunderstood by those skilled in the art without further description, itbeing borne in mind that numerous changes may be made in the detailsdisclosed without departing from the spirit of the invention.

1 claim:

1. A method of manufacturing a thermoplastic polymeric resin striphaving a foam core and an integral skin of the same material, comprisingcausing a foamable thermoplastic polymeric resin heated to a plasticstate and mixed with a blowing agent under pressure to flow alongradials of a laterally diverging die passageway to a transverselyarcuate die orifice curved in a plane parallel to the width of saidpassageway, continuously extruding and feeding a thin, narrow strip ofsaid resin from said die orifice at a feed rate at least of the order of40 feet per minute and expanding said resin to form a foam with a lowdensity of no more than about 3 pounds per cubic foot, immediatelychilling at least one surface of the extrudate, as it emerges from thedie, rapidly enough and to a temperature low enough to prevent expansionof said surface and to provide thereon a thin ,smooth,high-density skinof said material of thickness of the order of 0.001 to 0.005 inchmolecularly integral with the expanded foam, said chilling beingcommenced at a location spaced from the die orifice a distance of theorder of 0.015 to 0.050 inch, and continuously spreading the chilledextrudate laterally over an arcuate mandrel as the extrudate is fed fromsaid die orifice and while the extrudate is expanding, to prevent theformation of wrinkles, thereby to form an expanded extrudate strip ofthe order of 0.020 to 0.300 inch thick and a maximum of about 18 incheswide.

2. A method in accordance with claim 1, wherein said spreading comprisesbiaxially orienting the molecules of said skin.

3. A method in accordance with claim 1, wherein said spreading isaccomplished by passing the extrudate over a mandrel spaced about 2 to 3inches from said die.

4. A method in accordance with claim 1, wherein the blowing agent isvolatile, and wherein said spreading is accomplished while said blowingagent remains under sufficient pressure to provide plasticity for saidextrudate.

2. A method in accordance with claim 1, wherein said spreading comprisesbiaxially orienting the molecules of said skin.
 3. A method inaccordance with claim 1, wherein said spreading is accomplished bypassing the extrudate over a mandrel spaced about 2 to 3 inches fromsaid die.
 4. A method in accordance with claim 1, wherein the blowingagent is volatile, and wherein said spreading is accomplished while saidblowing agent remains under sufficient pressure to provide plasticityfor said extrudate.